1. Field of the Invention
The present invention relates to an internal combustion engine fitted with a sensor arranged to detect knocking. The present invention also relates to a straddle-type vehicle equipped with the engine.
2. Description of the Related Art
An internal combustion engine can cause knocking in some cases depending on its operating conditions. Knocking should be avoided as much as possible because it results in, for example, unusual noise and performance degradation of the internal combustion engine. Conventionally, it is known that a sensor to detect knocking, that is, a knock sensor, is fitted to an internal combustion engine. It is also known that, upon detecting knocking by the knock sensor, an action such as changing ignition timing is taken.
In order to detect knocking with high accuracy, it is preferable to dispose the knock sensor at a position near the location at which knocking occurs. JP 2004-301106 A discloses a water-cooled engine in which a knock sensor is fitted to a cylinder block.
A water-cooled engine needs a flow passage for coolant, i.e., a water jacket, to be formed in, for example, a cylinder block and a cylinder head. It also requires, for example, a pump for conveying the coolant and a radiator for cooling the coolant. For this reason, the structure of the water-cooled engine tends to be complicated.
A straddle-type vehicle equipped with a single-cylinder internal combustion engine (hereinafter referred to as a “single-cylinder engine”) is known, such as a relatively small-sized motorcycle. The single-cylinder engine has the advantage that it has a simpler structure than a multi-cylinder engine. To fully exploit the advantage, the single-cylinder engine has a relatively simple cooling structure. For that reason, conventionally, fins are provided on the cylinder block or the cylinder head so that at least a portion of the cylinder block or the cylinder head can be cooled by air.
In the air-cooled engine provided with fins, the cylinder block and so forth are cooled from the surface. On the contrary, in the water-cooled engine, the cylinder block and so forth are cooled from a water jacket disposed inside the surface. The knock sensor is disposed on a boss provided on the surface of the engine. This means that, when the boss is provided for the air-cooled engine provided with fins, engine cooling becomes insufficient, and consequently, cooling of the knock sensor may become insufficient. In other words, when the above-described conventional technique, in which it is assumed that cooling is done from the inside of the surface of the engine, is applied to the air-cooled engine, the temperature of the knock sensor may become too high, degrading the reliability of the knock sensor. In contrast, if the knock sensor is disposed at a location far from the location at which knocking occurs in order to dispose the knock sensor at a location at which the temperature is as low as possible, it will be difficult to detect knocking with high accuracy.